1. Field of the Invention
The present invention relates generally to high temperature heat pipes. It relates in particular to reusable, durable heat pipes which are capable of operating at temperatures up to about 3000.degree. F. in an oxidizing environment and has applications above 3000.degree. F. in an inert or vacuum environment.
2. Prior Art
High-temperature heat pipes have been fabricated and operated in oxidizing environments at temperatures below about 2700.degree. F. Most of these heat pipes use refractory metal containers which are coated externally with an oxidation resistant coating. The coating is usually a ceramic material which typically has a low tensile strength and a low coefficient of thermal expansion. The state-of-the-art in high-temperature heat pipes uses tungsten, molybdenum, or some other refractory-metal heat pipe with an external disilicide coating for oxidation resistance. The coating is very fragile and is limited in use beyond temperatures of about 2700.degree. F. The coating must withstand the thermal stresses produced from differences in thermal expansion of the refractory metal and the ceramic coating.
The disadvantages of the prior art are:
1. Currently, the maximum use temperature for heat pipes in an oxidating environment is 2700.degree. F.
2. Oxidation resistant coatings are fragile and susceptible to cracking, impact, thermal stress, and erosion problems. Once the coating is damaged, the refractory-metal heat-pipe container is exposed and can oxidize very rapidly at temperatures above 2700.degree. F.
a. For leading edge and nose cap applications on hypersonic vehicles, this could lead to a catastrophic vehicle failure.
b. For waste heat recovery applications, this could severely limit the life of the system.
3. Structural loads are withstood entirely by the refractory metal heat-pipe container. The ceramic coating serves no structural function other than oxidation resistance. Since refractory metals are typically very heavy compared to ceramic materials, the resulting heat pipe designs are heavy; and mass is a very important consideration in many applications, especially in hypersonic vehicles.
4. Ceramic coatings have very low thermal conductivities and, hence, degrade the efficiency of the heat-pipe in rejecting heat. Outer surface temperatures are a function of the overall thermal resistance of the heat pipe system.
Accordingly, it is the primary object of the present invention to develop a reusable, durable heat-pipe which can operate at temperatures up to about 3000.degree. F. in an oxidizing environment and has applications at temperatures above 3000.degree. F. for insert or vacuum environments. Applications of this invention are many and include: cooling stagnation regions of hypersonic vehicles (leading edges and nose caps), cooling nozzle and throat areas of jet and rocket designs, waste heat recovery from nuclear and fossil fuel plants, and thermally inert structures such as space antennas, mirrors, laser platforms, and telescopes. The primary application for the present invention is to cool leading edges and nose caps of hypersonic vehicles, which requires the use of an oxidation resistant surface.